The present invention is directed to the art of elastomer fluid seals and, more particularly, to compression-type elastomer seals. Still more particularly, the present invention is directed to elastomer seals arranged in an annular configuration to form a compression-type annular seal.
Elastomer seals arranged in an annular configuration are used in a variety of applications, for example, in oilfield equipment such as casing hangers, tubing hangers, and wellhead valves. Typically, the annular seals are exposed to axially directed compressive stress to create a strain-induced fluid seal about an inner diameter seal face or an outer diameter seal face or both. Elastomers, however, are subject to certain relaxation phenomena which tend, over time, to diminish the sealing capacity of a compression-type elastomer seal.
Elastomer extrusion is a relaxation phenomenon resulting from a physical reaction of the elastomer over a period of time to stress. In elastomer extrusion, the viscous flow property of elastomers gradually reduces contact stress at the seal faces, resulting in reduced pressure sealing capacity with respect to the sealed fluids. Stress relaxation is a relaxation phenomenon characterized by decaying stress over a period time in response to a breakdown of primary chemical bonds within the elastomer. Stress relaxation operates independently of extrusion to diminish seal strain at the inner and outer seal faces.
It is generally agreed that the relaxation phenomena occur at rates proportional to logarithmic time when the elastomer seal is used within conservative temperature limits. Thus, a compression-type elastomer seal may offer a reasonable life span in a relatively controlled environment, particularly where seal compression is adjusted periodically to maintain a reasonably constant axial stress. The environment of oilfield equipment, however, can never be wholly controlled and rigorous temperature conditions have been demonstrated to accelerate the relaxation processes. Excessive temperatures, for example, can cause relaxation processes to occur at rates that are proportional to linear time, rather than the much slower logarithmic time of a controlled environment.
Hence, it would be useful to devise a means for diminishing the detrimental effect of the above-described relaxation processes, whereby the sealing capacity of an annular elastomer seal is improved.